The final drive ratio is the single number that ties engine RPM to road speed in any given gear: it is the transmission's gear ratio multiplied by the axle ratio. This calculator finds that number, turns it into the engine RPM produced at any speed for a given tire, and lets you compare axle ratio options side by side before committing to a re-gear.

How the final drive ratio works

Every gear in a vehicle multiplies through to the wheels as transmission gear ratio x axle ratio. The transmission ratio changes with the gear selected — low gears (numerically high, like 3.36:1) multiply torque for acceleration, while an overdrive top gear (numerically below 1.0, like 0.70:1) trades torque for speed and lower engine RPM. The axle ratio is fixed by the differential's ring-and-pinion gears and applies the same in every transmission gear. Multiplying the two together in top gear gives the number most people mean when they say "final drive ratio" or "rear end gear" — for example a 0.70 top gear behind a 4.10 axle gives a 2.87:1 final drive, even though the axle itself is numerically tall.

Turning a ratio into RPM at speed

A final drive ratio alone does not tell you how fast the engine spins at a given road speed — that also depends on the tire's overall diameter, since a bigger tire covers more ground per revolution. The RPM at Speed tab uses the standard formula RPM = (final drive x mph x 336) / tire diameter (in), where 336 is a constant that folds in the conversion from miles per hour and inches to wheel revolutions per minute. A taller tire or a numerically lower ("taller") final drive both lower RPM at a given speed; a shorter tire or a numerically higher final drive both raise it.

Choosing a final drive: acceleration vs. cruising RPM

Re-gearing a differential is a real trade-off. A numerically higher axle ratio (say 4.10 instead of 3.55) multiplies more torque at the wheels for quicker acceleration and stronger towing, but it raises engine RPM — and fuel use and noise — at every cruising speed. A numerically lower axle does the opposite: relaxed, quiet, economical highway RPM at the cost of off-the-line punch. The Ratio Comparison tab is built for exactly this decision: hold the transmission gear and tire constant, list your candidate axle ratios, and read off the resulting final drive and RPM at your usual highway speed for each.